Question

How can the pitch of sound produced in piano be changed

Answer 1

There are no ways to change the pitch of the notes, outside of varying the string tuning. Perhaps a severe change in temperature will have that effect by altering the tension, but its not something musically useful, as each string will react differently.
However, some synthesizers have tone bending levers, where you can alter the pitch, up or down by jiggling the lever. And many electric pianos have controls where you can change the pitch of the whole keyboard up or down.

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Answer 2

A shorter, lighter string, under more tension, vibrates faster, and produces a higher-pitched sound..... At some point in the bass, and this is not standardized from one make of piano to another, the number of strings per note changes from three or two then for the lowest notes, one string per note.